The present invention relates generally to fuel cells and, more specifically, to fuels cells, electrode assemblies, and electrodes that comprise silicon substrates and/or sol-gel derived support structures, as well as to methods relating thereto.
A fuel cell is an energy conversion device that consists essentially of two electrodes, an anode and a cathode, and an electrolyte that is interposed between the anode and cathode. Unlike a battery, fuel cell reactants are supplied externally rather than internally. Fuel cells operate by converting fuels, such as hydrogen or methanol, to electrical power through an electrochemical process rather than combustion. It does so by harnessing the electrons released from controlled oxidation-reduction reactions occurring on the surface of a catalyst. A fuel cell can produce electricity continuously so long as fuel is supplied from an outside source.
In electrochemical fuel cells employing methanol as the fuel supplied to the anode, the electrochemical reactions are essentially as follows: first, a methanol molecule""s carbon-hydrogen, and oxygen-hydrogen bonds are broken to generate electrons and protons; simultaneously, a water molecule""s oxygen-hydrogen bond is also broken to generate an additional electron and proton. The carbon from the methanol and the oxygen from the water combine to form carbon dioxide. Oxygen from air supplied to the cathode is reduced to anions with the addition of electrons. The ions formed at the anode and the cathode migrate through the interposing electrolyte and combine to form water. Thus, the electrochemical reactions of a direct methanol fuel cell (DMFC) are as follows:
With respect to state-of-the-art fuels cells, electrode assemblies, and electrodes, several different configurations and structures have been contemplated. For example, numerous attempts have been made to construct fuel cells and electrode assemblies that utilize a solid polymer electrolyte (SPE) as an integral part of the electrode assembly (hence, the term membrane electrode assembly (MEA) has been coined). A significant problem, however, with DMFCs utilizing solid polymer electrolytes is a phenomenon known as xe2x80x9cmethanol crossover.xe2x80x9d As is depicted in FIG. 1, methanol in conventional DMFCs has a tendency to cross-over from the anode to the cathode via diffusion (i.e., it migrates through the electrolyte), where it adsorbs onto the cathode catalyst and reacts with oxygen from the air resulting in a parasitic loss of methanol fuel and concomitant reduction in fuel cell voltage. Indeed, performance losses of 40-100 mV at a given current density have been observed at the cathode of DMFCs utilizing a direct methanol feed (Potje-Kamloth et al., Abstract No. 105, Extended Abstracts 92-2, xe2x80x9cFall Meeting of the Electrochemical Societyxe2x80x9d (1992), (Kuver et al., J. Power Sources 52:77 (1994)).
Exemplary solid polymer electrolyte DMFCs include those that have recently been developed by NASA""s Jet Propulsion Laboratory (JPL). A detailed description of such JPL fuel cell designs may be found, for example, in U.S. Pat. No. 5,523,177 to Kosek et al., U.S. Pat. No. 5,599,638 to Surampundi et al., U.S. Pat. No. 5,773,162 to Surampundi et al., and U.S. Pat. 5,945,231 to Narayanan et al. Although the teachings associated with these patents have arguably advanced the art, the various membrane electrode assemblies (MEAs) disclosed therein do not eliminate the problem of methanol cross-over.
Other attempts for reducing methanol cross-over in solid polymer electrolyte DMFCs include structural modifications of the central solid polymer membrane. Exemplary in this regard are the MEAs disclosed in U.S. Pat. No. 4,664,761 to Zupancic et al. (discloses proton-conducting membrane made of an interpenetrating polymer network), U.S. Pat. No. 5,672,438 to Banarjee et al. (discloses proton-conducting laminated membrane), and U.S. Pat. No. 5,919,583 to Grot et al. (discloses proton-conducting membrane that includes an inorganic filler). Although the various MEA designs disclosed in these patents are able to reduce methanol cross-over to some degree, they nevertheless still have relatively high methanol permeabilities.
In addition to methanol cross-over, another significant problem with state-of-the-art fuel cell designs (especially solid polymer electrolyte DMFC designs) is catalytic inefficiency. For example, conventional solid polymer electrolyte DMFC designs generally attempt to maximize the surface contact between the catalyst and the solid polymer electrolyte. In this regard, it is reportedly crucial to maximize the three-phase interface that exists between the catalyst, the solid polymer electrolyte membrane, and the reactants (that permeate through the solid polymer electrolyte); such a three-phase boundary is reportedly needed to enhance efficiency and electrical capacity. As a result, a primary objective of previous DMFC research has been to optimize catalyst use by maximizing the surface area of catalyst in contact with the solid polymer electrolyte (catalyst not in direct contact with the solid polymer electrolyte has been termed xe2x80x9cnon-reactingxe2x80x9d catalyst).
Thus, conventional methods for fabricating high-surface-area electro-catalytic electrodes for use with solid polymer electrolyte DMFCs generally include: (1) depositing on the surface of a solid polymer electrolyte either a porous metal film, a planar distributions of metal particles, or carbon supported catalyst powders; (2) embedding metal grids or meshes into the surface of a solid polymer electrolyte; or (3) embedding catalytically active components into the surface of a solid polymer electrolyte. All of these conventional methods employ traditional electrocatalyst deposition techniques such as, for example, electroplating, sputtering and metal evaporation. As such, these methods generally result in catalyst loadings in excess of 0.4 mg/cm2. A conventional state-of-the-art electrode assembly is shown in FIG. 2A, and a conventional catalyst utilization scheme is shown in FIG. 2B (wherein the three-phase interface between the catalyst, the membrane, and the reactants are shown). As shown in FIG. 2A, an exemplary conventional state-of-the-art electrode assembly 200 consists essentially of a graphite block 202 (that functions as a current collector and as a flow field), an interposing Teflon mask 204, a porous anode 206, a catalyzed membrane 208 (with embedded catalyst particles), a porous cathode 210, a second interposing Teflon mask 212, and a graphite block 214, all of which are sandwiched together. The conventional fabrication techniques and materials associated with making such state-of-the art fuel cells are not generally amenable to miniaturization or mass production.
Although significant progress has been made with respect to these and other fuel cell problems, there is still a need in the art for improved fuels cells, electrode assemblies, and electrodes. The present invention fulfills these needs and provides for further related advantages.
In brief, the present invention is directed fuels cells, electrode assemblies, and electrodes that comprise silicon substrates and/or sol-gel derived support structures, as well as to methods relating thereto. In one embodiment, the present invention is directed to an electrode assembly adapted for use with a fuel cell, wherein the electrode assembly comprises: an anode derived from a first planar silicon substrate; an electrolyte; a cathode derived from a second planar silicon substrate; wherein the anode and the cathode are spaced apart and substantially parallel to each other so as to define a spaced apart region (or an interstitial region), and wherein the electrolyte is interposed between the anode and the cathode. The first and second planar silicon substrates may be silicon wafers (n-type, p-type, doped, or nondoped). In addition, the electrode assembly may further comprise a blocking media that is substantially impermeable to at least methanol and is substantially permeable to hydrogen atoms, wherein the blocking media is interposed between the anode and the cathode. The blocking media may be located anywhere within the spaced apart region; however, it is preferably integrally connected to the cathode. The blocking media may comprise a metallic membrane, and the blocking media may comprise palladium, niobium, tantalum, vanadium, or various combinations thereof. The blocking may even comprise a plurality of proton conducting plugs.
The anode of the electrode assembly may have a plurality of etched or micromachined flow channels (for delivering a hydrogen or hydrocarbon fuel), and may have a plurality of porous regions wherein each of the plurality of porous regions contains a solid porous rectangular region having a volume of about 3xc3x9710xe2x88x924 cm3. In addition, the plurality of porous regions of the anode may be nanoporous, mesoporous, and/or macroporous, and may comprise an ordered or random array of parallel pores. In addition, the plurality of porous regions of the anode may contain anode pore surfaces, wherein the anode pore surfaces have a catalyst thereon. The catalyst may comprise a plurality of noncontiguous chemisorbed metallic particles; and the catalyst may be a chemisorbed bi-metallic catalyst derived from platinum and ruthenium percursors.
The cathode of the electrode assembly may have a plurality of etched or micromachined flow channels (for delivering oxygen or air), and may have a plurality of porous regions that may be nanoporous, mesoporous, and/or macroporous, and may comprise a random array of sponge-like interconnected pores having an open cell structure. In addition, the plurality of porous regions of the cathode may contain cathode pore surfaces, wherein the cathode pore surfaces have a catalyst thereon. The catalyst may comprise a plurality of noncontiguous chemisorbed metallic particles; and the catalyst may be a chemisorbed metallic catalyst derived from platinum percursors.
The electrolyte of the electrode assembly may comprise a solid polymer electrolyte such as, for example, a perfluorosulfonic polymer membrane. In addition, the anode pore surfaces having a catalyst thereon, may further include at least a portion of the electrolyte thereon, wherein the electrolyte may be a solid polymer electrolyte that has a thickness ranging from about 0.05 to about 0.5 microns. Similarly, the cathode pore surfaces having a catalyst thereon, may also further include at least a portion of the electrolyte thereon, wherein the electrolyte may be a solid polymer electrolyte that has a thickness ranging from about 0.05 to about 0.5 microns. Still further, the electrolyte may comprise a first and second solid polymer electrolyte coating and an acid, wherein the first solid polymer electrolyte coating is on the anode, and wherein the second solid polymer electrolyte coating is on the cathode, and wherein the acid is contained in an organic fuel that flows through the anode and the spaced apart region.
The organic fuel may comprise water and an alcohol selected from the group consisting ethanol, propanol, methanol, or a combination thereof, and the acid may be phosphoric acid, sulfuric acid, or a combination thereof. In addition, the organic fuel may be equal molar amounts of methanol and water together with the acid in amount of about 0.25 M.
In another embodiment, the present invention is directed to an electrode assembly adapted for use with a fuel cell, wherein the fuel cell comprises: an anode derived from a first planar silicon substrate, wherein the anode has integrally associated therewith a plurality of anode sol-gel derived support structures; an electrolyte; a cathode derived from a second planar silicon substrate, wherein the cathode has integrally associated therewith a plurality of cathode sol-gel derived support structures; wherein the anode and the cathode are spaced apart and substantially parallel to each other so as to define a spaced apart region, and wherein the electrolyte is interposed between the anode and the cathode. This embodiment of the present invention is inclusive of all of the various aspects and features associated with the above-described non-sol-gel electrode assembly and need not be repeated here.
The present invention is also directed to an electrode adapted for use with a fuel cell, wherein the electrode comprises a silicon substrate that functions as a current conductor, wherein the silicon substrate has a plurality of pores that define pore surfaces, wherein at least a portion of the pore surfaces have a catalyst thereon, wherein the catalyst is derived from one or more metallic precursors chemisorbed onto at least the pore surfaces.
The present invention is also directed to an electrode assembly adapted for use with a fuel, comprising: anode derived from a first planar silicon substrate; an electrolyte; a cathode derived from a second planar silicon substrate; wherein the anode and the cathode are spaced apart and substantially parallel to each other so as to define a spaced apart region, and wherein the electrolyte is interposed between the anode and the cathode, and wherein the anode and the cathode each have a plurality of porous regions that are separated from one another by a plurality of nonporous silicon regions and that define anode pore surfaces and cathode pore surfaces, and wherein the anode pore surfaces and the cathode pore surfaces each have a catalyst dispersed thereon such that the catalyst is noncontiguously dispersed throughout the plurality of porous regions of the anode and the cathode, and wherein the anode and the cathode each have been doped such that the anode and the cathode are each capable of serving as a current conductor.
The present invention is also directed to an electrode adapted for use with a fuel cell, wherein the fuel-cell comprises a sol-gel derived support structure that functions as a current conductor, wherein the sol-gel derived support structure has a plurality of pores that define pore surfaces, wherein at least a portion of the pore surfaces have a catalyst thereon, wherein the catalyst is derived from one or more metallic precursors chemisorbed onto at least the pore surfaces.
The present invention is also directed to a hydrogen and/or hydrocarbon fuel cell that comprises any of the above-described electrodes and/or electrode assemblies.
These and other aspects of the present invention will become more evident upon reference to following detailed description and attached drawings. It is to be understood that various changes, alterations, and substitutions may be made to the teachings contained herein without departing from the spirit and scope of the present invention. It is to be further understood that the drawings are illustrative (hence, not to scale) and symbolic of exemplary embodiments of the present invention.